1. Field of the Invention
This invention relates to conveyors and, more particularly, to those of the powered roller type in which rollers are powered by means of a power-driven endless belt which, by frictionally engaging the underside of the rollers, causes them to rotate.
This invention relates further to means for regulating the propelling power transmitted to the rollers and thus to control the accumulation pressure exerted against an imposed obstacle, such as a case stop or an accumulation of stalled packages on the conveyor line, which pressure often is sufficient to disrupt the operation or cause damage to the contents of the packages.
2. Description of the Prior Art
In powered roller conveyors, regulation of the propelling power has been attempted by lifting the load, and sometimes the carrying rollers supporting the load, out of contact with the driving belt. This requires a substantial amount of costly energy.
Another means for regulating the propelling power has been to retract the driving means so that it is out of contact with the load. This means usually requires considerable energy to actuate and, if this energy must be supplied by the kinetic energy of the load, it follows that it is not responsive to light loads or mixed loads involving even a small percentage of light-weight articles.
In conveyors of the belt-driven live roller type, articles are propelled by a powered belt held in frictional engagement with the load-carrying rollers. The propelling power is directly proportional to the pressure of the belt against the rollers.
In the conventional belt-driven live roller conveyors, snubber rollers are used, located midway between a pair of load-carrying rollers and so positioned that the belt weaves a serpentine path and has a substantial wrap-around on both snubber and carrying rollers. Belt pressure is regulated by adjusting the belt tension, usually by means of a belt take-up mechanism. To obtain sufficient propelling power, belt must be tightened sufficiently to propel the heaviest anticipated load; actually, the belt must be tightened excessively to compensate for the expansion and contraction of the conveyor elements due to changes in temperature or humidity, and other factors, including permanent belt stretch and wear on the components. In addition to requiring frequent costly adjustments of numerous components, the labyrinth path the belt traverses frequently causes belt-tracking problems. Due to the extreme tension required in the belt, some components, such as the snubber-roller and pulley bearings, must be more durable and more costly than would otherwise be required.